Mineral-fiber insulation is well known and has been a commercial product for a long period of time. The insulation is made from rock or slag fibers often referred to as mineral wool or rockwool. Fibrous mineral-wool insulation products generally comprise fibers bonded together by a cured thermoset polymeric binder to impart rigidity and improved mechanical performance to the insulation.
Mineral fibers have not been linked to disease in man. On the other hand, research has established that asbestos fibers when inhaled can cause significant disease in man. Though the exact mechanisms responsible for the biological activity of inhaled asbestos fibers is unknown, it is widely believed that their ability to remain in the lung for extended periods of time is an important factor. The durability or residence time in the lung of mineral fibers, however, appears to be much less than that of asbestos fibers.
Nonetheless, as an added precaution the German government has proposed regulations for mineral fibers. The proposed German regulations indicate that mineral wools should contain very high amounts of alkali and alkaline earth oxides to give the fibers a very high biosolubility. In particular, the proposed regulations set forth, as a criterion for sufficient biosolubility, a KI index where the sum of CaO+MgO+Na.sub.2 O+K.sub.2 O+BaO+B.sub.2 O.sub.3 minus two times the Al.sub.2 O.sub.3 must be at least 40 weight percent (i.e., KI=.SIGMA.(CaO+MgO+Na.sub.2 O+K.sub.2 O+BaO+B.sub.2 O.sub.3)-2Al.sub.2 O.sub.3 .gtoreq.40). Mineral-fiber compositions meeting this criterion are considered to be free of suspicion.
An undesirable feature of highly soluble mineral-wool compositions having a KI value of at least 40, however, has been that they have low viscosities at the forming temperature. Thus, a problem for the manufacturer is to produce fibers which meet the index and other desired criteria: fibers should meet the KI index, be fiberizable in standard rockwool processes, have sufficient durability, and have acceptable insulating properties. Another problem has been to achieve mineral fibers having a KI value of at least 40 and high-temperature resistance.
An important use of mineral-fiber insulation is as a fire barrier in buildings. To perform properly in this application, the mineral fiber must have high-temperature resistance. Most mineral wools with high-temperature resistance contain high amounts of iron oxides (4 to 12 weight percent). The iron oxide acts as a nucleating agent so that the fiber will crystallize rather than melt during a fire. This high amount of iron oxide also reduces the high-temperature viscosity of the melt and can make fiber forming difficult. This is particularly true for compositions formulated with very high amounts of CaO+MgO to give a high biosolubility.